1. Field of the Invention
The present invention relates to a device and a method for reading information stored in a phosphor layer wherein an excitation radiation is emitted in the direction of a transparent carrier material having disposed thereon the phosphor layer to be read, the phosphor layer in response emits an emission radiation and the emission radiation emitted by the phosphor layer is received.
In particular for medical purposes, an image is generated of an object, for example a patient, using x-rays, wherein the image is stored in a phosphor layer as a latent image. To read the x-ray image stored in the phosphor layer, the phosphor layer is excited by a radiation source. This excitation by the radiation sauce causes the phosphor layer to emit light having an intensity corresponding to the x-ray image stored in the phosphor layer. The light emitted by the phosphor layer is received by a receiver, so that the x-ray image stored in the phosphor layer can be made visible. The x-ray image can, for example, be shown directly on a monitor. Alternatively, the x-ray image can also be recorded on a photographic x-ray film specifically designed for capturing x-ray images.
The phosphor layers are typically applied to a carrier material which may be either transparent or reflective. If the carrier material is reflective, then the radiation source and the receiver are arranged on the same side of the carrier material, i.e., on the side of the carrier material on which the phosphor layer is applied.
If the phosphor layer is arranged on a transparent carrier material, then the radiation source is positioned on one side of the carrier material, while the receiver are positioned on the opposite other side of the carrier material. This arrangement is advantageous in that a larger portion of the radiation emitted by the excited phosphor layer can be received by the receiver. As a result, the x-ray image stored in the phosphor layer is reproduced with a better quality.
2. Description of the Related Art
U.S. Pat. No. 4,953,038 discloses a device for reading a phosphor layer arranged on a transparent record carrier. In the disclosed device, a light source is positioned on that side of the transparent record carrier which does not include the phosphor layer. The light source illuminates a partial area of the phosphor layer. The record carrier together with the applied phosphor layer can be moved relative to the light source. The light source simultaneously illuminates a plurality of adjacent pixels of the phosphor layer which are arranged in the form of a two-dimensional array and excites the phosphor layer, causing the phosphor layer to emit light. The light emitted by the phosphor layer is captured by an optical fiber arrangement which is located on the side of the record carrier having the phosphor layer. The light collected by the optical fiber arrangement is subsequently conveyed to a charge coupled device, CCD. The CCD is used to detect an image of the information stored in the phosphor layer. When this method is used for reading the information stored in the phosphor layer, light emitted by the radiation source and exciting the phosphor layer may disadvantageously also be collected by the optical fiber arrangement and conveyed to the CCD. This tends to introduce noise in the x-ray image which is detected by the CCD. To prevent the introduction of noise, U.S. Pat. No. 4,953,038 proposes to place a filter in the optical path between the phosphor layer and the CCD, wherein the filter suppresses those wavelengths which are not a part of the radiation emitted by the phosphor layer. This arrangement prevents the light emitted by the radiation source from reaching the CCD. However, such filter disadvantageously tends to be relatively thick, with the filter thickness typically exceeding 0.3 mm. The light emitted by the phosphor layer may be scattered when passing through the filter. As a result, the CCD may not be able to detect the exact location from which the light is collected. In addition, radiation emitted by the phosphor layer may even be xe2x80x9clostxe2x80x9d when passing through the filter and may thus escape detection by the CCD. The visual representation of the x-ray image may therefore be distorted and of poor quality.
It is therefore an object of the present invention to provide a device and a method for reading information stored in a phosphor layer which reproduce the information with an improved quality.
The object is solved by disposing a reflective layer in the optical path between the radiation source and the receiver for reflecting at least a portion of the excitation radiation.
According to one aspect of the invention, at least a portion of the excitation radiation used to excite the phosphor layer is reflected. For this purpose, a reflective layer capable of reflecting the excitation radiation is placed between the radiation source and the receiver. Such reflective layers have typically a thickness of xcex/4 and are therefore much thinner than conventional filter layers. xcex refers here to the wavelength of the excitation radiation which is to be predominantly reflected by the reflective layer. The construction of the reflective layer depends on the spectral characteristics of the excitation radiation and the desired reflected wavelength. The reflective layer can be designed for a specific application. Alternatively, several reflective layers, which may be designed for different wavelengths to be reflected may be arranged in the optical path between the radiation source and the receiver. The reflective layers are of conventional design and are advantageously prepared in the form of so-called xe2x80x9cthin layers.xe2x80x9d Reflective layers of this type are described, for example, in the optical treatise xe2x80x9cContemporary Optics for Scientists and Engineersxe2x80x9d by Ellen Nussbaum et al., Prentice-Hall, Inc., Englewood Cliffs, N.J. 1976, pp. 182 to 198, and in the treatise xe2x80x9cTechnische Optikxe2x80x9d by Prof. Gottfried Schrxc3x6der, Vogelbuch-Verlag, Wxc3xcrzburg, 6. Edition, pp. 108 to 110.
The reflective layer need not be designed to completely reflect the entire excitation radiation emitted by the radiation source. Rather, the reflective layer may be tailored to certain wavelength regions of the excitation radiation. For example, wavelength regions of the infrared spectral region which do not contribute to excitation of the phosphor layer may be removed from the excitation radiation before the excitation radiation impinges on the phosphor layer.
It may also be possible, as disclosed in U.S. Pat. No. 4,953,038, to arrangexe2x80x94in addition to a reflective layerxe2x80x94a filter in the optical path between the phosphor layer and an imaging means for absorbing the excitation radiation. The imaging means for imaging the radiation emitted by the phosphor layer may be implemented, for example, in the form of an optical fiber arrangement. Such an optical fiber arrangement has the additional advantage over the arrangement disclosed in U.S. Pat. No. 4,953,038 that the filter layer can be made thinner, so that a greater portion of the radiation emitted by the phosphor layer can be collected and imaged on the receiver. Because scattering of the emitted radiation is also reduced, the image is sharper and of higher quality.
According to an advantageous embodiment of the invention, the phosphor layer includes a special phosphor with a special crystalline needle-shaped structure. A phosphor of this type is known, for example, from the European patent application EP 0 751 200 A1. This special phosphor has a plurality of xe2x80x9cneedlesxe2x80x9d which guide both the excitation radiation and the emitted radiation. The phosphor is prepared by growing crystalline xe2x80x9cneedlesxe2x80x9d having a base area of approximately between 1 and 25 xcexcm2 and a height corresponding to the desired layer thickness of the phosphor layer. Excitation radiation impinging normal to the surface of the phosphor layer is guided in each xe2x80x9cneedlexe2x80x9d essentially without being scattered until reaching a color center in the crystal lattice in which the information is stored. The emitted radiation produced through excitation of the color center is guided in the respective xe2x80x9cneedle.xe2x80x9d Depending on the reflective index between the xe2x80x9cneedles,xe2x80x9d the respective emitted radiation is a reflected and guided out of the xe2x80x9cneedle.xe2x80x9d By using this special phosphor, scattering of the excitation radiation inside the phosphor carrier is practically eliminated. It particular, with information stored in the phosphor layer being read out row-by-row, scattering of the excitation radiation perpendicular to the row direction can be detrimental, since color centers can also be excited which are located in a row of the phosphor layer different from the row currently being read. Consequently, image radiation can xe2x80x9cget lost,xe2x80x9d i.e., cannot be detected by the receiver. Scattering of the emission radiation within the phosphor carrier layer is also eliminated, so that the emitted radiation can be detected by the receiver with excellent spatial resolution.
The reflective layer arranged between the phosphor layer and the receiver reflects the excitation radiation which passes through the transparent carrier and the phosphor layer and reintroduces the excitation radiation into the phosphor layer. As a result, the color centers in the phosphor layer are once more excited and emit radiation. Accordingly, the phosphor layer emits an increased amount of radiation which can then be detected by the receiver. This reproduces the information with better quality and enhanced sharpness.
According to another advantageous embodiment of the invention, a reflective layer for reflecting the emission radiation emitted by the phosphor layer is arranged between the radiation source and the phosphor layer. This reflective layer reflects the emission radiation, which is emitted by the phosphor layer towards the side of the phosphor layer facing away from the receiver, towards the receiver. The reflected radiation can thus also be detected by the receiver, further improving the reproduction of the information.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of.